Did you completely setup layman? You need to have followed the instructions in the document that mahdi1234 linked for you, particularly in section 2 where the line "source /var/lib/layman/make.conf" is added to make.conf. If you did this and still can't install from overlays, check that the line was added to your actual in-use make.conf (it could be either /etc/make.conf or the newer /etc/portage/make.conf), and it might help to post your emerge --info if you still have no luck.